Equity-Based Awards, Restricted Unit Awards (Details) - Restricted Common Unit Awards [Member] - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate of restricted common unit awards		25.00%
Summary of awards activity, equity instruments other than options [Roll Forward]
Beginning of period (in units)		682,294	1,960,520	4,229,790
Vested (in units)		(681,044)	(1,234,502)	(2,009,970)
Forfeited (in units)		(1,250)	(43,724)	(259,300)
End of period (in units)		0	682,294	1,960,520
Restricted units outstanding, weighted-average grant date fair value [Roll Forward]
Weighted-average grant date fair value per unit, at beginning of period (in dollars per unit)	[1]	$ 28.61	$ 27.88	$ 26.96
Vested weighted-average grant date fair value per unit (in dollars per unit)	[1]	28.60	27.45	26.00
Forfeited weighted-average grant date fair value per unit (in dollars per unit)	[1]	$ 31.07	28.48	27.53
Weighted-average grant date fair value per unit, at end of period (in dollars per unit)	[1]		$ 28.61	$ 27.88
Summary of awards activity, equity instruments other than options, additional disclosures [Abstract]
Cash distributions paid to restricted common unitholders		$ 0.3	$ 1.6	$ 4.0
Total intrinsic value of restricted common unit awards that vested during period		$ 18.9	$ 28.5	$ 67.3

[1] Determined by dividing the aggregate grant date fair value of awards (before an allowance for forfeitures) by the number of awards issued.